1. Field of the Invention
The present invention relates to a photoconductive element, as exemplified by an element for generating/detecting (which means an element capable of at least one of generation and detection) an electromagnetic wave in a frequency region from millimeter wavebands to terahertz wavebands (30 GHz to 30 THz). More specifically, the present invention relates to an optical semiconductor element for generating/detecting an electromagnetic wave pulse containing a Fourier component in the frequency region by optical pulse irradiation, and to a terahertz time domain spectroscopy apparatus using the optical semiconductor element.
2. Description of the Related Art
In recent years, non-destructive sensing technology using an electromagnetic wave ranging from millimeter waves to terahertz (THz) waves (30 GHz to 30 THz; hereinafter, also referred to simply as terahertz wave) has been developed. In the application field of the electromagnetic wave in this frequency band, imaging technology for safer fluoroscopy has been developed as an alternative to X-ray imaging. Other developing technologies include spectroscopic technology for determining absorption spectrum or complex permittivity inside a substance to examine physical properties such as the bonding state of molecules, measurement technology for examining physical properties such as the carrier density or mobility and the conductivity, and analysis technology for biomolecules.
A photoconductive element has been widely used for generating/detecting a terahertz wave. The photoconductive element includes a particular kind of semiconductor having relatively high mobility and sub-picosecond carrier lifetime, and two electrodes provided thereon. The photoconductive element is structured in a manner that, when a gap between the electrodes is irradiated with ultrashort-pulsed laser light under the application of voltage between the electrodes, a current flows between the electrodes instantaneously by excited photocarriers, and thereby a terahertz wave is emitted therefrom. The above-mentioned measurement and imaging technologies have been studied with the use of such photoconductive element also as a terahertz wave detector to constitute a terahertz time domain spectrosope device (THz-TDS).
In general, a titanium sapphire laser is used as an ultrashort-pulsed laser for excitation. There is, however, a demand for the use of a fiber laser at a telecommunication wavelength for reduction in size and cost. Since the wavelength in this case is 1 μm or more, low temperature grown (LT-)GaAs, which has heretofore been used as a photoconductive element, becomes a transparent medium and it cannot be used. LT-InGaAs is therefore being studied as an alternative photoconductive material (see Japanese Patent Application Laid-Open No. 2006-086227 and U.S. Patent Application Publication No. 2006/0134892).
However, since an InGaAs system has a smaller band gap than that of GaAs, the carrier density of an intrinsic semiconductor is higher, and in addition the number of crystal defects is apt to increase because InGaAs is ternary. Also owing to the increased residual carrier density, it is difficult to increase the resistance. Therefore, the application voltage cannot be increased as compared to GaAs and it is difficult to increase the differential change of photocarriers, which limits terahertz wave generation efficiency. The use of an InGaAs system as a detector increases a dark current to increase noise, which leads S/N ratio degradation. As countermeasures, another technology of using a semiconductor heterostructure as a photoconductive layer so as to trap generated carriers to thereby increase the resistance is being considered (see Optics Express, vol. 16, p. 9565 (2008)).
In the semiconductor heterostructure element described in Optics Express, vol. 16, p. 9565 (2008), however, a heterointerface deteriorates when a low temperature grown film is exposed to high temperature, and it is difficult to increase the carrier trapping effect as designed. Japanese Patent Application Laid-Open No. 2006-086227 also describes that the In composition is adjusted to be smaller than that of a lattice-matched system (0.45). With such value, the band gap is widened and the increase in resistance can be expected, but the degree of lattice mismatch with a substrate becomes larger. If the thickness is reduced in order to suppress lattice defects, the absorption amount of excited light is reduced and the amplitude of the terahertz wave is reduced.